Corundum is a crystalline form of aluminum oxide and is found in various different colors, most of which are generally referred to as sapphire. Sapphire is a hard and strong material with a hardness of 9.0 on the Mohs scale, and, as such, is capable of scratching nearly all other minerals. Due to its brittle nature, it is susceptible to dramatic strength reductions as a result of small defects caused by interactions with its environment. Therefore, while a sapphire part may be very reliable leaving the factory, after time and use it may become less reliable due to an accumulation of minor damage.
Additionally, producing defect free parts can be very challenging. Brittle materials' strength are limited by the flaw population on the surface. An inconsistent or inadequate surface finishing can lead to very weak parts. Glass is chemically strengthened to a significant depth to minimize the effect of these flaws, but on extremely hard materials such as sapphire, a similar process is not readily available. For example, the sapphire's hardness makes cutting and polishing the material both difficult and time consuming when conventional processing techniques are implemented. Further, conventional processing tools such as cutters experience relatively rapid wear when used on sapphire. This further increases the resource demand when surface finishing sapphire parts.
Additionally, the use of parts formed from different materials, such as sapphire and glass, glass and plastic, sapphire and plastic, and so on, in devices can lead to differences in the optical appearance of side by side components due to the high reflectance of sapphire. Between two materials having particularly strong differences in reflectance the effect can be very noticeable.